Ceramic product based on lithium aluminum silicate

ABSTRACT

A ceramic product containing at least 70% lithium aluminium silicate of 100% total mass weight percent; and a non-wetting agent in the form of barium sulphate. Most preferably, the lithium aluminium silicate includes petalite in finely divided form which has been fired to a temperature between 1150° C. and 1210° C.

TECHNICAL FIELD

This invention relates to a ceramic product, and then particularly onefor handling molten aluminium. In this specification the term ‘ceramicproduct’ refers to a product which has been formed by firing a ceramicraw material to a suitable temperature.

BACKGROUND ART

Molten aluminium, which has a relatively low melting point ofapproximately 700° C., is difficult to handle and for this reason theapparatus used usually comprises a product that has a high thermal shockresistance, a high resistance to weftability, a low thermalconductivity/capacity, and high strength.

None of the products which has hitherto been used for the aforesaidpurpose, amongst others aluminium titanate; fused silica; cast iron andfibrous ceramic board, has ideal properties in this regard.

OBJECT OF THE INVENTION

It is an object of this invention to provide a ceramic product which theapplicant believes has advantages over the known arrangements.

DISCLOSURE OF THE INVENTION

According to the invention a ceramic product is provided which containsat least 70% lithium aluminium silicate on a mass per mass basis.

Lithium aluminium silicate (hereafter referred to as LAS) occursnaturally as the mineral petalite, and is used in small quantities inceramic formulations instead of feldspar in order to decrease thethermal expansion of such formulations, and hence improve theirresistance to thermal shock. It is also used in glass compositions forstove tops and oven ware.

Applicant has now found that an LAS-ceramic product according to theinvention has extremely low thermal expansion and can hence as such beused in any apparatus subjected to any form of heat exposure, such asfor example, that occurring in the handling of molten aluminium.

Such an LAS-ceramic product can be manufactured by forming a suitablequantity of finely divided petalite in any of the conventional methodssuch as slip casting; extrusion or injection moulding; and wet or drypressing and then firing it at a temperature in the order of between1150° C. and 1210° C.

For use in the aluminium industry, such an LAS-ceramic product does nothave to be fired to full density. It has been found that fired densitiesof the order of 60 to 70% of the theoretical are obtained when firing atbetween 1150 and 1210° C. At these densities, the product shows nodimensional changes, no thermal expansion, adequate strength, a lowthermal conductivity and thermal capacity, and also takes an excellentcoating when painted with protective materials such as boron nitride, asis normal practice in the aluminium industry. It can furthermore beimpregnated with liquid substances intended for enhancing themetallurgical properties of the product.

If required, the density of the LAS-ceramic product can be lowered bythe addition of fly ash microspheres and/or a suitable combustiblematerial.

Further according to the invention the LAS-ceramic product according tothe invention includes a non-wetting agent which has been incorporatedin the fired LAS-ceramic product.

The reason for such incorporation is that the inherent wettability ofLAS by molten aluminium poses severe problems when the protectivecoating usually provided on the LAS-ceramic product is removed, whichmay, for example, happen during use of the ceramic product.

It has furthermore been found that such wettability cannot be reduced bythe incorporation of known non-wetting agents in the ceramic formulationof the product according to the invention, because such agents tend toreact with the LAS during the firing process causing them to lose theirability as non-wetting agents.

Furthermore, by incorporating the non-wetting agent in the firedLAS-ceramic product, the thermal expansion properties of the ceramicproduct are not adversely affected.

Further according to the invention the non-wetting agent comprisesbarium sulphate (BaSO₄), which is a well known non-wetting agent in theceramic industry.

Further according to the invention a method for improving thenon-wettability of an LAS-ceramic product is provided which includes thestep of providing a suitable non-wetting agent such as BaSO₄ in thefired LAS-ceramic product.

In one form of the invention the BaSO₄ may be provided in the firedLAS-ceramic product by impregnating the semi-porous fired LAS-ceramicproduct with a saturated aqueous solution of barium sulphide (BaS)which, after drying, is oxidised in situ to BaSO₄.

Such oxidation may be effected by calcining the dried product to500-800° C. in an oxidising atmosphere.

Commercial BaS can be used, or BaS can be prepared by the reduction ofBaSO₄ with carbon at 1190° C., or with ammonia at 1000° C.

In another form of the invention the BaSO₄ may be provided byimpregnating the semi-porous fired LAS-ceramic product with a solutionof BaCl₂, and then treating the product with sulphuric acid to causeBaSO₄ to precipitate inside the pores of the ceramic product.

Preferably the BaCl₂-solution is a substantially saturated one, and thesulphuric acid comprises diluted sulphuric acid.

The BaCl₂—solution can either be prepared by dissolving BaCl₂ in water,or by treating BaCO₃ with hydrochloric acid.

In a preferred form of the invention the ceramic product according tothe invention comprises substantially 100% LAS (mass per mass).

BEST MODE FOR CARRYING OUT THE INVENTION

A semi-porous LAS-ceramic product according to the invention comprising100% (mass per mass) LAS is provided by forming a suitable quantity ofpetalite in finely divided form by means of any one of the conventionalmethods such as slip casting; extrusion of injection moulding; or wet ordry pressing, and firing the formed product at a temperature of between1150° C. and 1210° C.

Preferably said finely divided petalite comprises a mixture of twofractions:

a first one having an average particle size in the order of 1micrometer, used in a concentration from 50 to 100% (mass per mass); and

a second one having a particle size finer than 75 micrometer, used in aconcentration of from 0-50% (mass per mass).

The non-wettability of said LAS-ceramic product is improved according tothe invention by incorporating BaSO₄ as non-wetting agent in the productin any one of the following two methods:

BaS—Impregnation

(i) A solution of BaS is prepared by intimately mixing one mass part ofBaSO₄ with half a mass part of carbon, and firing the mixture at 1190°C. for 2 hours;

(ii) One mass part of the product of step (i) is added to four massparts water; boiled for 12 hours; and the solids filtered off;

(iii) The solution resulting from step (ii) is kept at boiling point,and the semi-porous LAS-ceramic product immersed in the boiling solutionfor 15 minutes;

(iv) The impregnated product is air-dried and then calcined at 500-800°C. for 3 hours.

2. BaCl₂—Impregnation

(i) A saturated solution of BaCl₂ is prepared by dissolving 59 g BaCl₂in 100 ml H₂O at 100° C. (Alternatively 1 kg BaCO₃ in 0.95 kg H₂O istreated with 1.11 kg HCl (33% concentration by volume) to form asaturated solution of BaCl₂);

(ii) A diluted solution of H₂SO₄ (concentration not critical, say 50%(volume by volume) is prepared;

(iii) A cooled down semi-porous as fired LAS-ceramic product isimpregnated with the boiling BaCl₂—solution;

(iv) The ceramic product is cooled down to cause BaCl₂ to precipitateout in the pores of the ceramic product;

(v) Any excess water is removed from the ceramic product under vacuum;

(vi) The ceramic body is impregnated with the diluted H₂SO₄ which reactswith the BaCl₂ to cause BaSO₄ to precipitate inside the pores of theceramic product;

(vii) The ceramic product is dried and calcined to 700° C. to remove allvolatiles.

In both instances an LAS-ceramic product is formed which issubstantially non-wettable with molten aluminium.

It will be appreciated that the invention also includes within its scopean LAS-ceramic product of which the non-wettability has been improved bythe incorporation therein of BaSO₄ by any one of the aforesaid twomethods.

Results Obtained with a Product According to the Invention

The following results were obtained comparing an LAS-ceramic productaccording to the invention with products conventionally used in thehandling of molten aluminium.

In compiling these results, the importance to the process of therelevant refractory property is rated from 1-10, and then multiplied bythe rating (1-10) of the specific ceramic material in terms of theproperty, in order to generate a normalised rank for the differentmaterials in terms of their total performance.

1. Competitive products Aluminium Titanate (AT) Fused Silica (FS) CastIron (CI) Fibrous Ceramic Board (CB) Ceramic LAS-Product (LAS) 2.Weighted properties Thermal Shock Resistance (10) Resistance to Wetting(8) Thermal Conductivity/Capacity (5) Strength (5) 3. Material ratings3.1 Thermal Shock (×10) AT 10 (100) FS 10 (100) CI 10 (100) CB 6 (60)LAS 10 (100) 3.2 Wettability (×8) AT 10 (80) FS 4 (32) CI 0 (0) CB 10(80) LAS 10 (80) 3.3 Thermal Conductivity/Capacity (×5) AT 3 (15) FS 8(40) CI 0 (0) CB 10 (50) LAS 10 (50) 3.4 Strength (×5) AT 10 (50) FS 6(30) CI 10 (50) CB 2 (10) LAS 10 (50) Total Ratings Aluminium Titanate(AT) (245) = ±9 Fused Silica (FS) (207) = ±7 Cast Iron (CI) (150) = ±5Ceramic Board (CB) (200) = ±7 Ceramic LAS-product (LAS) (280) = ±10

It is clear from the above results that when used for the handling ofmolten aluminium, an LAS-ceramic product according to the invention hasmuch better overall properties than those of the aforementioned knownproducts.

It will also be appreciated that there are no doubt many variations indetail possible with a ceramic product according to the inventionwithout departing from the scope of the claims.

What is claimed is:
 1. A ceramic product, containing at least 70%lithium aluminum silicate (LAS) of total mass weight percent; and anon-wetting agent in the form of barium sulphate.
 2. The ceramic productof claim 1, wherein the lithium aluminum silicate comprises petalite infinely divided form which has been fired to a temperature of between1150° C. and 1210° C.
 3. The ceramic product of claim 2, which comprises100% lithium aluminum silicate of 100% total mass weight percent.
 4. Theceramic product of claim 3, wherein the non-wetting agent in the form ofbarium sulphate is incorporated in the LAS-ceramic product after beingfired.
 5. The ceramic product according to any one of the precedingclaims having a theoretical density of less than 70%.
 6. A method ofimproving the non-wettability of the ceramic product of any one ofclaims 1 to 3, which method includes the step of providing thenon-wetting agent in the form of barium sulphate in the LAS ceramicproduct after being fired.
 7. The method of claim 6, wherein the bariumsulphate is provided in the fired LAS-ceramic product by impregnatingthe semi-porous fired LAS-ceramic product with a saturated aqueoussolution of barium suiphide (BaS) which, after drying, is oxidised insitu to BaSO₄.
 8. The method of claim 7, wherein the oxidation iseffected by calcining the dried product to 500-800° C. in an oxidisingatmosphere.
 9. The method of claim 6, wherein the BaSO4 is provided byimpregnating the semi-porous fired LAS-ceramic product with a solutionof BaCl₂, and then treating the product with sulphuric acid to causeBaSO₄ to precipitate inside the pores of the ceramic product.
 10. Themethod of claim 9, wherein the BaCl₂—solution is a substantiallysaturated one, and the sulphuric acid comprises diluted sulphuric acid.11. A method of forming the ceramic product of any one of claims 1 to 3,which includes the steps of firing petalite in finely divided form to atemperature of between 1150° C. and 1210° C.; and impregnating the firedproduct with the non-wetting agent.
 12. The method of claim 11, whereinthe product is formed by means of any one of the following methods: slipcasting; extrusion or injection moulding; wet or dry pressing.
 13. Themethod of claim 11, wherein the finely divided petalite comprises amixture of two fractions: a first one having an average particle size of1 micrometer, used in a concentration from 50 to 100% (mass per mass);and a second one having an average particle size 75 micrometer, used ina concentration of from 0-50% (mass per mass).